This invention relates to improved fluidized-bed or slurry-type air-depolarized electrodes, and to electrochemical processes and apparatus, especially power sources, utilizing such electrodes.
Many modern batteries and fuel cells utilize oxygen from the air and/or other reactants which are not readily soluble in the battery or fuel cell electrolyte and which can otherwise not be easily supplied to the active sites of the so-called electrode catalyst at which they must electrochemically react for satisfactory operation of said batteries or fuel cells. This limited solubility of the reactant, and hence its restricted access to the active catalyst sites, severely limits the rate at which the insoluble or poorly soluble reactant can be electrochemically consumed, and hence the current density and power density of the battery or fuel cell.
Another serious drawback of many of these electrochemical power sources is that the active catalyst sites at the electrode surfaces at which said poorly soluble reactants can usefully react may be subject to inactivation by poisoning or clogging. Once these sites are inactivated, the electrode stops functioning, and the entire battery or fuel cell becomes inoperative.
It is an object of my invention to substantially increase the current density and hence the power density of such electrodes, especially oxygen-consuming electrodes, so as to permit them to meet high power requirements, e.g., in the propulsion of electric vehicles.
It is another object of my invention to increase the lifetime of such electrodes, and of batteries of fuel cells utilizing same, by providing the means to replace said electrode catalyst without dismantling or otherwise tampering with the rest of the power source system.
It is yet another object of my invention to render said active catalyst sites more effective in a functioning battery, fuel cell or other electrochemical system, device or process, so as to permit the use of a relatively inexpensive catalyst, such as activated carbon or rare earth cobaltite, instead of platinum or other noble metals, and so as to reduce the electrode polarization and thereby increase the overall energy efficiency of the battery, fuel cell or electrochemical process.
It is still another object of my invention to apply the concept of fluidized-bed or slurry-type electrode to air-consuming electrochemical systems and processes in a practicable and advantageous manner.
The use of slurry-type electrodes in hydrogen- and oxygen-consuming fuel cells has been shown to result in considerable improvements in current density. However, the slurry-type oxygen-depolarized electrodes disclosed heretofore have consisted of a three-phase mixture of electrolyte, catalyst suspension, and gaseous bubbles. Such a three-phase mixture presents severe practical problems, especially in air-depolarized electrodes, which have rendered the concept of fluidized-bed or slurry-type electrodes inapplicable thus far to fuel cells and other air-breathing systems. In particular, the direct injection of air into an electrolyte-catalyst mixture would result in entrainment of electrolyte and catalyst by the more than four volumes of nitrogen which must accompany each volume of oxygen consumed, and which must be rejected or allowed to escape from the system for satisfactory continuous operation. Moreover, the bubbles and foaming associated with a large rate of air flow through the electrolyte would interfere with ionic conductivity and thereby give rise to ohmic losses.